Chromium plating process with a pure nickel strike



United States Patent 3,186,925 CHRQMIUM PLATING PRGCESS WITH A PURENECKEL TREKE John A. Knshner, Detroit, Mich, assignor to Generai MotorsCorporation, Detroit, Mich, a corporation of Delaware No Drawing. FiledNov. 1, 1960, Ser. No. 66,417 3 Claims. ((31. 204-41) This inventionpertains to electroplating and more particularly to the forming of adecorative chromium plated part having improved durability.

Among the plating sequences currently being commercially used to producedecorative chromium plated parts is that involving the use of a duallayer nickel coating immediately subjacent the chromium coating.Although the dual layer nickel coating may be applied di- '\J;ectly ontoa basis metal, a more frequent commercial :pfaptice includes applying athin preliminary coating of a metal, such as copper, to the basis metaland then applying glhB dual layer nickel and chromium coatings. Morespecifically, the latter sequence involve initially applying a layer ofcopper to the surface of the basis metal. A layer of semi-bright nickelis then applied to the copper layer; a layer of fullabright nickel isapplied to the semibright nickel layer, and then an overlay ofdecorative chromium is applied to the full-bright nickel layer.

A not uncommon problem present in either of the above-mentionedsequences lies in obtaining satisfactory adhesion of the semi-brightnickel layer to the surface upon which it is applied. This problem isespecially significant when the semi-bright nickel layer is applied to acopper plated surface. The nature of the semi-bright plating bath andits inherent method of use are believed to be the cause of the pooradhesion of the nickel to a basis metal such as copper .or steel. Thepoor adhesion contributes to unsatisfactory durability, particularlyunsatis factory corrosion resistance, of the finished part. Althoughvarious explanations as to the cause of the unsatisfactory adhesion havebeen oilered heretofore, not one has provided a solution by whichsatisfactory adhesion can be consistently obtained under commercialproduction conditions.

Principal objects of the invention are to provide an improved dccorativechromium plated part which has improved durability due to improvedadhesion of the dual layer nickel coating and to provide an improvedmethod of forming a decorative chromium plated part.

The invention comprehends applying an extremly pure layer of nickel tothe surface of the part immediately prior to applying a conventionaldual layer nickel coating. It is essential to the invention that thenickel layer be pure in the sense that there are relatively nocodeposited substances in the nickel layer, as is true in conventionalcommercially applied nickel layers. It is also of importance that nocurrent be passed through a pure nickel plating bath solution while thepart is being immersed therein.

It is believed that bipolar eifects, inorganic contaminants in thesemi-bright nickel plating bath, organic additives such as wettingagents, brighteners and the like in the semibright nickel plating bathall contribute to codeposition of other substances with the nickel tocause poor adhesion of the semi-bright nickel layer to a basis metalsuch as cop-per or steel. All of these undesirable factors are obviatedby electrodepositing a pure nickel layer onto the basis metalimmediately before the layer of the semibright nickel is deposited.

An extremely adherent, uniform pure nickel layer can be obtained byimmersing the part to be plated into a suitable nickel plating bathWithout current on, in contra- 'ice distinction to immersion into asemi-bright nickel plating bath with current on.

The specific bath composition which is preferred to apply the purenickel layer may vary, depending upon the apparatus used, preferredplating times, temperatures, composition of the part being plated, sizeand configuration of the part being plated, etc. However, for most applications, particularly for copper plated parts, a Wattstype bathsolution, such as that having the following composition, can be used:

Bath I Nickel sulfate (NiSO -6H O) oz./gal 40 Nickel chloride (NiCl -6HO) oz./gal 4 Boric acid oz./gal 5 pH 3-3.5 Temperature F Cathode currentdensity amps/sq. ft 10-60 The bath is operated with air agitation and ananode to cathode area ratio of about 1:1. The purity of the bath ismaintained with constant recirculating filtration through an activecarbon pack and periodic dummying. It is preferred that the pH of thebath solution be maintained at approximately 2.0 to 4.0 for bestresults. However, in some instances, a pH as high as approximately 5.2may be preferred.

As previously indicated, the bath is used to apply an extremely purenickel coating. The bath therefore contains no materials other than asformulated above and, therefore, contains no wetting agents orbrighteners of any sort. Although a Watts-type bath, such as Bath I, isthe most generally satisfactory bath which can be used to apply my purenickel layer, in some instances it may be preferred to employ otherbaths to obtain the pure nickel coating. A high chloride bath such asfollows can be used:

Bath 11 As indicated in connection with Bath I, Bath 11 is used asdescribed above Without conventional addition agents and its purity imaintained by constant recirculating filtration through an active carbonpack and periodic dummying. Hence, substantially all impuritiesintroduced into the bath by drag-out from preceding baths aresubstantially continuously removed. Since the bath is formulated so asto be substantially free of contaminants, obviously the onlycontaminants introduced into it would be from drag-out of baths used inpreceding treatments, e.g., cleaning or copper plating. An anode tocathode area ratio of 1:1 is preferred and air agitation can be used.

Other nickel plating solutions which might be employed to attainsatisfactory results in certain instances would be as follows:

Bath III Nickel chloride (NiCl 6H O) .oz./gal 32 Boric acid oz./gal 4 pH0.9-1.1 Temperature F 130 pH 3.0-4.5 Temperature F 130 Cathode currentdensity amps./sq. ft. 50-100 Mild agitation, such as air agitation, canbe used. An anode to cathode area ratio of 1:1 can be used for the lowercurrent density depositions while a ratio of 3:1, respectively, ispreferred for higher current density depositions. Baths 111 and IV, likeBaths I and II, are used as described without conventional additionagents and while maintaining purity through constant recirculationthrough an activated carbon pack and periodic dummying. V

The pH of Baths I and II is preferably adjusted with HCl or H 80 whilethe pH of Bath III is preferably adjusted with HCl. The pH of Bath IVcan be adjusted with HCl, H 80 or HP. Since it is the presence offoreign metal salts and organic additives which principally causeimpurities in the coating to result, any of the common mineral acidswill at least be generally satisfactory for adjusting the pH of the bathsolution.

The specific manner in which the purity of the nickel plating baths ismaintained is not particularly critical and the purity can be maintainedin the known and accepted manner for purifying plating solutions. Asindicated above, constant recirculating filtration through an activatedcarbon pack can be used to remove organic contaminants. Periodicadditions of hydrogen peroxide can be used for the same purpose.Inorganic contaminants can be precipitated chemically but it is morefeasible to remove them under commercial production conditions byperiodic dummying of the bath.

As a specific example of the practice of my invention a metal part, suchas one made from SAE 1010 steel, is polished and cleaned to form asuitable surface for subsequently applied electrodeposited coatings. Theparticular manner in which the metal part is cleaned is only as materialto my invention as it is to any general electroplating procedure andmay, therefore, be varied considerably.

In one method of cleaning, the workpiece is immersed for four minutes tofive minutes in an alkaline soak cleaner containing two ounces pergallon to six ounces per gallon of the following'mixture, by weight:

Percent Sodium metasilicate 30 Trisodium phosphate 50 Soda ash 13 Sodiumresinate 7 line spray rinse, such as described above, and subjected toan anodic electrocleaning in a solution made up as follows:

Oz./ gal. Sodium orthosilicate 8 Tetrasodium pyrophosphate 1 Sodiumcarbonate 1 Using a bath temperature of approximately 160 F. to 180 F.the workpiece is subjected to an anodic potential for about 30 secondsto 45 seconds with an anode current density of approximately amperes persquare foot to 100 amperes per square foot. After the anodic cleaning,the part is thoroughly rinsed in cold water and then immersed forapproximately 30 seconds in a room temperature aqueous bath containing0.2% to 0.5% sulfuric acid,

by volume.

Thereafter the part is removed from the sulfuric acid solution,thoroughly rinsed and placed in a copper strike bath, such as one havingthe following composition:

Copper cyanide, CuCN 4- oz./ gal. Sodium carbonate, Na CO 2 oz./ gal.Rochelle salts 3 oz./ gal.

pH About 11.5.

Free sodium cyanide 0.9 oz./ gal. to 1.6 oz./ gal.

Temperature F.

Using this bath a copper coating of approximately 0.00005 inch inthickness is deposited in about two minutes when using a cathode currentdensity of about 60 amperes per square foot.

After the desired thickness of copper is deposited in the strikesolution, the workpiece is removed therefrom and directly placed in acopper plating bath having the following composition:

A copper coating approximately 0.0005 inch to 0.0008 7 inch in thicknessis deposited with both mechanical and air agitation of the bath under acathode current density of about 10 amperes per square foot to 30amperes per square foot and a bath temperature of about 160 F.

Thereafter the part is removed from the copper plating 3 bath, rinsed incold water and cleaned for approximately one-half minute in theabove-identified anodic electroclea'ning solution. It is then removedfrom the electrocleaning solution, rinsed in cold water and dipped in anaqueous solution containing about 1% to 3%, by volume, sulfuric acid.The workpiece is then rinsed in water, preferably de-ionized water.

From the rinse the workpiece is directly immersed without current oninto a solution for plating a pure nickel layer. A Watts-type bathsolution, such as Bath I can be used. The pure nickel layer is depositedas described in connection with Bath I to form a coating approximately0.00001 inch in thickness.

After the desired thickness has been obtained, the part is rinsed incold water and directly immersed, with current on, into a solution forplating a semi-bright nickel layer having a columnar microstructure. Asolution which can be used to obtain such a coating is as follows:

The nickel is deposited until a thickness of approximately 0.0007 inchis obtained. It is preferred to both introduce the workpiece into thesolution as well as remove it therefrom with the current on. In generalI have found best results to be obtained if the semi-brightbath containssulfur-free brighteners After the workpiece is removed fromthesemi-bright nickel plating solution it is rinsed and immersed, withcurrent on, into a full-bright nickel plating solution, such as onehaving the following composition:

NiS0 -6H O g./l 300 NiCl -6H O g./l 40 HgBOg g./l 40 Sodium laurylsulfate g./l V 1 Coumarin g./l 0.3 Benzene mono-sulfonate g./l 10 B 0 tomake one liter.

.pI-l 3.2 Temperature F Cathode current density a.s.f 30

For both nickel plating solutions cast carbon anodes bagged in nylon canbe employed. After nickel plating, if desired, the parts can becathodically cleaned, rinsed and dipped in an aqueous solutioncontaining 30%, by weight, hydrochloric acid.

The workpiece is then rinsed in cold water and immersed in a chromiumplating bath to apply the chromi um coating onto the dual layer nickelcoating. A chro mium plating bath that can be used to provide excellentresults is one containing 32 ounces per gallon chromic acid (ClG and inwhich the chromic acid to sulfuric acid ratio is about 100:1. The bathis operated at a temperature of approximately 118 F. and a cathodecurrent density of approximately 180 amperes per square foot.Sufi'icient chromium is deposited to apply an overlay of at least0.000015 inch. As is typical for chromium deposition, no agitation isrequired during the deposition and an anode to cathode area ratio ofabout 1: 1-2:1, respectively, can be used.

The chromium overlay can also be accomplished in accordance with theinvention described in the previously led United States patentapplication serial No. 833,672, nowcabandoned, in the names of FrederickL. Brower and Robeft C. Miller and which is assigned to the assignee ofthe present invention. The invention described in the referenced UnitedStates patent application entails employing a pulsating current densityto apply the chromium overlay. The cathode current density isperiodically raised and lowered in cyclic fashion repeatedly during thechromium deposition to obtain a more uniform coverage on the surface ofthe part.

As previously indicated, a steel part such as one made from SAE 1020steel can be decoratively chromium plated without the use of an initialcopper layer. In such instance my pure nickel layer is directly appliedto the steel, preferably using Bath 11 in the manner described. The partis then rinsed in cold water and the conventional dual layer nickelcoating and chromium overlay applied in the same manner as hereinbeforeindicated in detail.

The pure nickel layer which is used need only have an average thicknesswhich is sufiicient to produce a continuous coating on the surface beingplated. Even on parts having complex surface contours, a continuouscoating can generally be achieved with a pure nickel layer having anaverage thickness of about 0.0001 inch. In someinstances, however, itmay be preferred to employ a pure nickel layer having a greater averagethickness.

It is to be understood that although the invention has been described inconnection with certain specific examples thereof no limitation isintended thereby except as defined in the appended claims.

What is claimed is:

1. In a method of decorative chromium plating which h" includes thesteps of treating a workpiece in at least one bath solution to preparethe workpiece surface for reception of a nickel strike,electrodepositing a nickel strike to the workpiece surface,electrodepositing a semibright nickel coating onto tne nickel strike,electrodepositing a bright nickel coating onto the semibright nickelcoating and electrodepositing an overlay of chromium onto the brightnickel coating, the improvement which comprises electrodepositing thenickel strike from a bath which consists of at least one inorganicnickel ion-producing salt, an inorganic pH adjusting agent and water andwhich is maintained substantially free of organic and other inorganicmaterials introduced into the bath by drag-out from any precedingsurface preparation baths by substantially continuous purificationmeans.

2. The method as defined in claim 1 wherein the workpiece is immersed inthe nickel strike bath without current on and the nickel strike isdeposited after the workpiece is fully immersed.

3. In a method of decorative chromium plating which includes the stepsof treating a workpiece in at least one bath solution to prepare theworkpiece surface for reception of a nickel strike, electrodepositing anickel strike to the workpiece surface, elcctrodepositing a semibrightnickel coating onto the nickel strike, electrodepositing a bright nickelcoating onto the semibright nickel coating and electrodepositing anoverlay of chromium onto the bright nickel coating, the improvementwhich comprises immersing the workpiece without current on in a nickelstrike bath which consists of nickel sulfate, nickel chloride, boricacid and water and which is maintained substantially free of organic andother inorganic materials introduced into the bath during its use bysubstantially continuous purification means and the nickel strike iselectrodeposited after the workpiece is fully immersed.

References Cited by the Examiner UNITED STATES PATENTS 1,371,414 3/21Edison 204-238 1,99 1,747 2/ 35 Hogaboom 2044 1 2,879,211 3 5 9 Kardoset a1. 20441 3,009,238 11/61 Wesley et al 204-41 FOREIGN PATENTS 684,43412/52 Great Britain. 789,887 1/58 Great Britain.

OTHER REFERENCES Rodgers: Handbook of Practical Electroplating MacmillanCo., New York, 1959, page 219.

JOHN H. MACK, Primary Examiner. JOHN R. SPECK, JOSEPH REBOLD, Examiners.

3. IN A METHOD OF DECORATIVE CHROMIUM PLATING WHICH INCLUDES THE STEPSOF TREATING A WORKPIECE IN AT LEAST ONE BATH SOLUTION TO PREPARE THEWORKPIECE SURFACE FOR RECEPTION OF A NICKEL STRIKE, ELECTRODEPOSITING ANICKEL STRIKE TO THE WORKPIECE SURFACE, ELECTRODEPOSITING A SEMIBRIGHTNICKEL COATING ONTO THE NICKEL STRIKE, ELECTRODEPOSITING A BRIGHT NICKELCOATING ONTO THE SEMIBRIGHT NICKEL COATING AND ELECTRODEPOSITING ANOVERLAY OF CHROMIUM ONTO THE BRIGHT NICKEL COATING, THE IMPROVEMENTWHICH COMPRISES IMMERSING THE WORKPIECE WITHOUT CURRENT ON IN A NICKELSTRIKE BATH WHICH CONSISTS OF NICKEL SULFATE, NICKEL CHLORIDE, BORICACID AND WATER AND WHICH IS MAINTAINED SUBSTANTIALLY FREE OF ORGANIC ANDOTHER INORGANIC MATERIALS INTRODUCED INTO THE BATH DURING ITS USE BYSUBSTANTIALLY CONTINUOUS PURIFICATION MEANS AND THE NICKEL STRIKE ISELECTRODEPOSITED AFTER THE WORKPIECE IS FULLY IMMERSED.